1. Field of the Invention
The present invention relates to EEG monitors. The invention further relates to an EEG monitor in two parts connected with electrical wires. The invention more specifically concerns an EEG monitor comprising power supply means. The invention further relates to a method for communicating between two parts of an EEG monitor.
EEG is the commonly used abbreviation for Electro Encephalo-Gram, which is generally speaking a method of electrically monitoring brain activity of a person. Systems for monitoring EEGs have been known for many years. However with the general technological development, EEG monitoring systems, which may be carried or worn continuously by a person to be monitored, have been devised.
It is known to measure EEG by placing electrodes on the scalp of a person, and to record and analyse the EEG signal for various diagnostic purposes.
2. The Prior Art
A system for such a use is known from WO-A1-2006/047874, which describes measurement of brain waves by use of electrodes placed in connection with at least one of the ears of the subject, i.e. placed on an outer ear part or placed in the ear canal. The measurements are used particularly for detecting the onset of an epileptic seizure. WO-A1-2006/047874 also describes the use of electrodes in pairs as detection and reference electrodes respectively, such a setup being well known in the field of electroencephalography.
EEG monitors may also be applied for surveillance of persons having diabetes, where blood sugar levels are monitored in order to warn against hypoglycemic attacks caused by low blood sugar levels. Hypoglycaemic attacks may lead to unconsciousness and even death. A system for such surveillance of an eminent hypoglycaemic attack is disclosed in WO-A-2006/066577. This is, however, an implanted subcutaneous system.
WO-A1-2007/047667 describes an ear plug for measuring EEG-signals. The ear plug comprises an exterior shell with electrodes, the shell being made of a soft, compressible material. The signals obtained with the ear plug are transmitted to external units for processing and monitoring.
Typically, a personal EEG monitor will be made in two parts as mentioned above, i.e. a base part with signal processing means and an electrode part with at least two electrodes for measuring the EEG signal of a person. Preferably the electrode part will be made as small as possible, such that it is easily attached to the skin surface on the head of the person. The base part often comprises the power supply means, and is therefore larger. The base part would typically be arranged in a less visible position. The two parts will be connected through wires. The electrodes for measuring the EEG signal are often prepared to be arranged with skin contact in the ear region of a person. The electrodes may also be capacitive.
Since the electrode part is adapted for being arranged at or in the ear or ear region of a person, it will often comprise a receiver or speaker applied for giving sound or voice messages to the user. This could be warnings about an imminent hypoglycemic seizure. But the receiver may be applied for any type of sounds.
The electrode part could also be applied for other types of transducers. Examples of such transducers in the electrode part could be a microphone for transforming sounds into electrical signals, or a temperature sensor or an accelerometer. Also other transducers could be considered relevant to arrange in the electrode part. The electrical signal from such a transducer needs to be transferred to the signal processing means of the base part of the EEG monitor, normally by an extra pair of wires, for further processing, logging or transmission to a remote device.
One problem in having such a transducer, e.g. a microphone, is that the wires used for transferring the signal from the transducer to the base part may pick up electromagnetic interference. The electrical signal generated in e.g. a microphone may be relatively weak, e.g. 1-5 μV, and therefore rather sensitive to noise.
This problem is larger when a receiver is arranged in the electrode part, since the wires supplying the receiver signal, which may be 2 V at peak level, will be arranged close to the wires transferring the signal from the EEG electrodes and e.g. from a transducer. Therefore, there may be a risk that the receiver signal will induce noise into the wires carrying the EEG signal and any transducer signal.
US-A1-2004/0116151 describes a databus which can be applied for a hearing aid between a base part and a peripheral component. This databus is described as needing transfer of power, clock and synchronization signal.
One problem is that the number of wires should be as low as possible in order to keep the total diameter of the bundle of wires connecting the two parts as small as possible. Each wire is connected both to the electrode part and to the base part, e.g. through a connector. This connection will take up some space, and will in general be a weak point in the construction, i.e. there is a risk of losing the electrical connection at this point. Furthermore the connectors typically applied are relatively expensive components. Therefore, keeping the necessary number of connections to a minimum is to be preferred.